Buoyant manoeuvrable ski

ABSTRACT

A BUOYANT MANOEUVRABLE ELONGATED SKI OF FOAM PLASTIC MATERIAL, HAVING AN UPTURNED SKI-NOSE AT THE FRONT, AND A FLAT, UNOBSTRUCTED BOTTOM SURFACE. A FOOT-WELL IS PROVIDED IN THE TOP OF THE SKI JUST TO THE REAR OF THE MIDFLOTATION POINT SUCH THAT THE FLOTATION MOMENT FORWARDLY OF THE FOOT-WELL IS FROM 10% TO 30% GREATER THAN THE FLOTATION MOMENT REARWARDLY THEREOF. THE WEIGHT MOMENT OF SKI REARWARDLY OF THE FOOT-WELL IS FROM 20% TO 40% GREATER THAN THE WEIGHT MOMENT OF THE SKI FORWARDLY THEREOF. A RESILIENT FLIPPER IS ATTACHED TO THE SKI PART-WAY UP THE SKI-NOSE AND EXTENDS DOWNWARDLY AND REARWARDLY FROM ITS POINT OF ATTACHMENT TO THE SKI.

P. TILINGS Feb. 20, 1973 BUOYANT MANOEUVRABLE SKI Filed May 15, 1971 IWEWOR PAUL TILINGS Agent United States Patent U.S. c1. 9 310 D 17 Claims ABSTRACT OF THE DISCLOSURE A buoyant manoeuvrable elongated ski of foam plastic material, having an upturned ski-nose at the front, and a flat, unobstructed bottom surface. A foot-well is provided in the top of the ski just to the rear of the midflotation point such that the flotation moment for-wardly of the foot-well is from to greater than the flotation moment rearwardly thereof. The weight moment of the ski rearwardly of the foot-well is from 20% to greater than the weight moment of the ski forwardly thereof. A resilient flipper is attached to the ski part-way up the ski-nose and extends downwardly and rearwardly from its point of attachment to the ski.

This invention relates to an improvement in sk-is or buoyant devices of the type which may be worn on a persons feet to support him on water for skiing, walking on the water, or a number of aquatic sports.

Before dealing with the prior art and the specific objects of the present invention, it will be helpful to discuss certain theoretical considerations which relate to the designing of buoyant skis which can also be used for walking on the water.

Firstly, the flotation of each buoyant ski should be sufiicient to support the weight of an average person. Naturally, since it is desirable to make the skis as small as possible, it is important not to over-size the skis and give them a greater flotation than the minimum necessary to support the average person. Generally speaking, a flotation of 150 pounds per buoyant ski has been found to be suflicient. Allowing for a ski weight of from 5 to 7 pounds using any well-known rigid cellular foamed plastic, it will require about 2.6 cubic feet of ski to provide the necessary 150 pounds of flotation.

The next consideration relates to the length of the skis. It has been found that sk-is should be at least about 4 /2 feet long, for otherwise the ski will pitch too noticeably when the user tries to walk. Naturally, a longer ski has a greater flotation moment arm and will resist pitch more effectively than a shorter ski. However, too great a length of ski means that the ski is too diflicult to turn in the water using the foot alone. Thus, a compromise length in the range of 4 /2 to 5 /2 feet is generally used in the prior art. Taking a length of about 4% feet, the average cross-sectional area would have to be in the neighbourhood of .55 square foot (80 square inches) in order to provide a flotation of 150 pounds minimum. To determine the dimensions of the cross-section of the ski, it is necessary to consider the requirements for using the ski as a water ski to be towed behind a power boat. Firstly, the ski should have a bottom that is either substantially flat or slightly concave (upwardly hollowed), and its width should be small so that, when the ski is planing, the water drag on the bottom will be minimized. Also, it is preferable that the flat bottom surface have parallel edges, because this has been found to improve the tracking ability of the ski. Thus, from the point of view of water drag on the bottom of the ski, the narrower the ski the better. However, in a float ing ski of a given cross-section, the narrower the bottom "Ice the deeper must be the ski, and if the ski is too deep and narrow, the user will find himself standing too far down into the water, and it will be very diflicult to manoeuvre the skis when it is desired to walk on the water. Thus, a compromise must be made, and a width of about 8 inches to 9 inches has been found to be satisfactory. With this width, the depth comes to about 9 inches to 10 inches for a ski of approximately rectangular cross-section, but it is possible to make the ski wider at the top than at the bottom with downwardly and inwardly sloping sides, so that the depth of ski can be reduced to about 8 inches.

The next consideration bears on an important failing of the prior art, and relates to the positioning of the footharness or foot-well on the ski. All of the prior art designs recognize the desirability of positioning the footharness or foot-well slightly rearwardly 0f the mid-flotation point, the latter being defined as the point about which the flotation moments at either end are equal. The reason for the desirability of positioning the foot-harness or foot-well rearwardly of the mid-flotation point is that when the user walks, his foot rocks forward with the toes down and the heel up, and this forward rocking of the foot tends to cause the ski to pitch forward. If the harness were centred on the ski so that the flotation moments at either end were equal, the user would have the sensation of falling or diving forward with the front end of the ski plowing beneath the surface, due to the forward rocking of his foot as he attempts to walk. Thus, it has been generally accepted in the prior art to position the foot-well or foot-harness rearwardly of the mid-flotation point by about 3 inches to 5 inches, and naturally this causes the forward flotation moment to exceed the rearward flotation moment. As mentioned above, this excess flotation moment to the front is desirable, but the concomitant thereof is that the forward end of the ski itself weighs more than rearward end as measured from the foot-harness. In other words, the weight moment to the forward end is greater than the weight moment to the rearward end. When the user tries to lift up on one ski to move it forwardly, the rear end comes out of the water first and the front end slopes down into the water, simply because the rear end is lighter. This does not cause problems as long as the user merely desires to move straight forward through the water. However, if the user desires to turn the ski in the water, he must force the partly submerged front end sideways through the water, and great resistance is met to this sideways motion.

It will now be helpful to turn to a discussion of the prior art, of which US. Pat. No. 3,134,114, issued May 26, 1964 to K. W. Stimm and entitled Buoyant Shoes, is exemplary. The Stimm patent describes a buoyant shoe of rigid foamed plastic material, the buoyant shoe having an upturned ski-end at the front, a foot-harness set slightly rearwardly of the mid-point, and a taper toward either end from the foot-harness. Stimms patent is primarily directed to the provision of downwardly and rearwardly sloping flippers extending at longitudinal intervals from the bottom surface of his buoyant shoe.

The Stimm patent has the following disadvantages, which it is the object of the present invention to overcome:

(a) Stimm provides a plurality of flippers extending from the bottom of his ski, and this has been found to produce great instability and wobble" when the buoyant shoe is used as a water ski and the user is towed behind a power boat at a speed greater than 10 miles an hour. It is extremely diflicult to keep the ski tracking straight. To overcome this disadvantage, it is necessary to remove the flippers and any other obstruction from the bottom riding surface of a water ski.

(b) Stimms tapering shape of the flat bottom surface of his buoyant shoe increases the difliculty of making the shoe track in a straight line through the water when being used as a water ski. It is far more preferable to have a bottom with parallel edges.

(c) From Stimms figures and description it is obvious that the front end of his buoyant shoe (the part forwardly of the foot-harness) has a greater volume than the part rearwardly of the foot-harness, and consequently it must weigh more. As pointed out above, such a construction is extremely disadvantageous when the user attempts to turn the ski when Walking on the water. When the user lifts up on the foot-harness, the rear end of the shoe comes out of the water, but the front end stays in the water, and it is very diflicult to force the partly submerged front end sideways in the water.

(d) Another disadvantage relates to the postioning of the flippers extending from the bottom surface of the buoyant shoe in the Stimm patent. When a person using the buoyant shoes attemps to step down on a shoe to get purchase on the water in order to push the other shoe forwardly, the downward push on the shoe which is intended to get purchase on the water causes an upward rush of the supporting water against the bottom of the shoe and around the sides as the weight is applied. This upward rush of supporting water first closes the flippers against the bottom of the buoyant shoe, and it is not until the user has moved the buoyant shoe at least a foot to the rear that the flippers actually open out and grab the water, thereby providing purchase. Thus, although the flippers of the Stimm patent do fulfill the function for which they are provided, considerable energy is wasted by the user as the shoe with which he attempts to get purchase on the water actually slides backwards before it grabs. Thus it is far more tiring to walk on the water with Stimms shoes than it would be using a ski which grabs the water immediately. It is tiring for the same reason that it is tiring to walk in soft sand or in soft, deep snow.

As mentioned above, the object of this invention is to provide a buoyant, manoeuverable ski free of the disadvantages enumerated above with respect to the Stimm patent.

It is also an object of this invention to provide a buoyant manoeuvrable ski for walking on the Water of which the rearward end is designed to provide a particularly good purchase on the water.

Accordingly, this invention provides a buoyant manoeuvrable ski comprising: an elongated body portion of rigid foam plastic material, the body portion having a forward end, a rearward end, and a bottom surface free of all obstructions, the bottom surface curving upwardly at the forward end to provide a ski-tip, foot-gripping means located between the ends of the body portion at a location rearwardly of the mid-flotation point such that the flotation moment forwardly of that location is from about to about 30% greater than the flotation moment rearwardly of that location, the weight moment of the ski rearwardly of the said location being from about 20% to about 40% greater than the weight moment of the complete ski forwardly of the said location, the ski having water-gripping means ensuring that the ski encounters less resistance to forward movement through water than to rearward movement through water.

One embodiment of this invention is shown in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which:

FIG. 1 is an elevational view of a buoyant ski constructed in accordance with this invention;

FIgIG. 2 is a plan view of the buoyant ski shown in FIG. 3 is an elevational view of the forward end of the buoyant ski shown in FIG. 1;

FIG. 4 is an elevational view of the rearward end of the buoyant ski shown in FIG. 1;

FIG. 5 is a sectional view taken at the line 55 in FIG. 1, drawn to a larger scale;

FIG. 6 is a sectional view taken at the line 6-6. in FIG. 1, drawn to the same scale as FIG. 5;

FIG. 7 is a sectional view taken at the line 77 in FIG. 1, drawn to the same scale as FIG. 5;

FIGS. 8, 9 and 10 are sequential views of the buoyant ski in use; and

FIG. 11 is a longitudinal vertical sectional view taken at the line 1010 in FIG. 2.

Turning now to FIG. 1, a buoyant manoeuvrable ski shown generally at 10 comprises an elongated body portion 12 having a forward end 14 and a rearward end 15. The body portion 12 has a bottom surface 16 free of all obstructions. At the forward end 14 the bottom surface 16 curves upwardly to provide a ski-tip 18. The bottom surface 16 has parallel sides.

Foot-gripping means in the form of a foot-well 20 is provided just rearwardly of the mid-point of the ski 10, and a fin 22 and flipper 24 complete the basic components of the ski 10.

Attention is now directed to FIG. 7, which shows the cross-section at the line 77 in FIG. 1. A first part 26 has a substantially square section, with a slight T shape at the top. First part 26 fits snugly into a U-shaped second part 28 having a bottom 16 which can be either flat as shown in solid line or concave as shown in broken lines, curving side walls 30, and a channel 32 in which the fin 22 is received. Sandwiched between the first part 26 and the second part 28 is a flat piece 34 of material such as plywood, which provides a reinforcing layer of rigid sheet material running substantially the whole length of the ski 10.

Turning to FIG. 6, it will be seen that the foot-well 20 is wholly contained within the first part 26, which is merely hollowed out down to the piece 34 of plywood, so that the foot of the user can rest directly on the plywood piece 34. The foot-well 20 has a forward toe-grip extension 35 into which the user can put the front part of his foot.

In FIG. 5 it can be seen that the front end of the first part 26 has a somewhat different cross-section, being more in the shape of a true T. The stem 36 of the T- section is narrower than the inside dimension of the U- section of the second part 28, such that two air spaces 40 are provided in the front half of the ski 10. The air spaces 40 extend between the dotted lines 42 and 43 in FIG. 1.

With the provision of the spaces 40, the ski 10 is approximately balanced about the fulcrum 44 (FIG. 1), prior to the addition of the flipper 24, the fin 22 and the weight 46 to be discussed below. The fulcrum 44 is the approximate point at which the upper part of the users foot lifts upwardly on the ski to raise it up in the water.

The first part 26 and the second part 28 are preferably made of a rigid foam plastic material, such as foamed or expanded polystyrene of approximately 2 pounds density, although other plastics well-known to those versed in this art may be used.

The fin 22 is well known on skis of this type, and need not be described in detail here. It can be made from polyethylene or from hardwood or plywood with good water-resistant characteristics. A satisfactory fin would be about 8 inches long, and extend roughly one inch below the bottom surface of the ski 10. In FIG. 7, the fin 22 is shown in solid and dotted outline, depending on whether the bottom surface 16 is flat or slightly curved.

Referring to FIGS. 1, 3, 4 and 11, the flipper 24 can be seen to be substantially rectangular in shape and slightly curved convex rearwardly. The flipper 24 is attached to the ski 10 at the ski-tip 18 along a line which is spaced about one-quarter of the distance from the bottom surface 16 to the top of the ski 10. When fixed in position as shown in FIG. 1, the free edge 48 extends below the bottom surface 16 of the ski by about the same distance; one-quarter of the depth of the ski 10. The flipper 24 is resilient. so that it can be forced. to lie along the bo tom surface of the ski 10, and when so lying, the free edge 48 is situated approximately at the point where the bottom surface 16 begins to curve up to form the ski-tip 18. Turing to FIG. 11, it will be seen that the attachment edge 50 of the flipper 24 has its marginal portion 52 bent at right-angles, and that the upcurving ski-tip 18 of the ski has a lateral slot 54 which is adapted to receive the marginal portion 52 of the flipper 24. Preferably, the marginal portion 52 is glued or otherwise securely fixed in the slot 54. It has been found that one suitable material for the flipper is polypropylene, and where the body of the ski itself is to be made of expanded polystyrene (Styrofoam), those skilled in this art will realize that glues suitable for polystyrene tend not to adhere to polypropylene (usually because of the smooth surface of the latter) whereas glues suitable for polypropylene usually dissolve or otherwise chemically change polystyrene. It is thus preferred to first toughen the surface of the polypropylene and then utilize a polystyrene-type glue.

In addition to securing the marginal portion 52 in the slot 54, it is possible to use threaded fastening devices, such as screws 56, to fasten the flipper 24 adjacent the marginal portion 52 securely through to the plywood piece 34.

Referring now to FIGS. 1 and 7, it will be seen that the ski 10 is provided with a small weight 46 adjacent the rearward end of the ski, the purpose of which is to cause the weight moment arm rearwardly of the fulcrum 44 to be from about to about 40% greater than the weight moment forwardly of the fulcrum 44. As mentioned in the earlier discussion of the theoretical considerations, it is necessary to have the rearward weight moment greater than the forward weight moment so that when the user lifts up at the fulcrum 44, the front end will come out of the water rather than the rearward end, and the user can then swing the front end from side to side to change the direction of the ski. The weight 46 can be of metal, wood, heavy plastic or other relatively dense material.

Attention is now directed to FIGS. 8, 9 and 10 which show three sequential positions as a ski is lifted and moved forwardly. FIG. 8 shows the normal attitude of the ski in the water when the user is merely standing in the skis and the skis are parallel and side-by-side. The ski is sloped approximately 15 to the horizontal in FIG. 8. In FIG. 9 the user has begun to move his right foot up wardly and forwardly ca rying the ski 60 with it. It will be noted that the forward end of the ski 60 comes well out of the water, that the flipper 24 is completely clear of the water, and that the weighted rearward end of the ski 60 remains partly immersed in the water. Because of the normal sloping attitude of the ski 60 in FIG. 8 (15 to the horizontal), the ski is already in a position to jump out of the water in the forward direction along the angle that it is lying in FIG. 8. The smooth bottom surface 16 and the roughened rearward end 15 (to be described below) are such that it is easier for the ski to slide along the smooth surface 16 in the direction in which the ski points than to move bodily upward with water flowing past the rearward end 15. FIG. 10 shows the further forward and downward motion when the user again presses downwardly on the ski after moving it forward the desired distance. Naturally, the ski immediately pivots in the clockwise direction about itsrearward end to bring the ski down flat into the water. While the ski 60 is pivoting in the clockwise direction, the water beneath the ski is being pushed out from beneath the ski in all directions, including forwardly along the surface. This forward splash of Water (arrows 62 in FIG. 9) catch the flipper 24 and pry it open to the substantially vertical position shown in FIG. 10 so that the moment the flipper enters the surface of the water, it is in a vertical position and instantaneously grabs the water and obtains a proper purchase from which the user can then bring the other ski forward.

When the buoyant manoeuvrable ski of this invention is being used for water skiing the ski tends to ride up on the top of the water and is said to plane, with the forward end of the ski raised up out of the water several inches. This raising of the front end of the ski is suflicient to keep the flipper 24 entirely clear of the water while the ski is being used for water skiing. Thus, no flippers or other troublesome obstructions occur along the flat bottom surface 16 of the ski 10 to hinder water skiing.

Attention is now directed to FIGS. 1, 2 and 4, for an understanding of the way in which the rearward end 15 of the ski 10 selectively recessed to permit the rearward end 15 to obtain an improved purchase on the water. As seen particularly in FIG. 4, the rearward end 15 is provided with two elongated rabbets 65 in the upper and lower segments, and three circular rabbets 66 in the central segment. Each rabbet 65 and 66 is in the form of a smooth depression or hollow in the rearward end 15. Also the rearward end 15 defines with he bottom surface 16 an actuate angle of about which is considered to help the rearward end 15 dig into'the water when the user urges rearwardly on the ski. The specific arrangement of rabbets is not critical.

A test was conducted with two skis identical except for the rabbetting 65 and 66. One ski was provided with the rabbets as shown in FIG. 4, and the other ski had a flat rear surface without rabbets. Both skis had the identical slope on the rearward end 15. To test each ski, the ski was loaded with pounds of ballast while floating in the water, and a steady force of 10 pounds was applied to the ski to drag it rearwardly.

The length of time taken to drag each ski a distance of 16 feet was recorded. To drag the ski provided with the 5 spoon-like rabbets 65 and 66 as shown in FIG. 4 a distance of 16 feet required 10 seconds, whereas only 6 seconds were required to pull the flat-back ski the distance of 16 feet. This test shows the excellent grip on the water provided by the rabbets 65 and 66. Thus, the rabbets 65 and 66 and flipper 24 work together to provide an excellent purchase or grip on the water, when the ski has reached the situation shown in FIG. 10, where the user is about to place a rearward force on the ski as he brings the other ski up to and past the ski shown.

An example will now be given to show the calculation of flotation moment arms and of weight moment arms. Let us take a ski as shown in FIG. 1, and calculate the flotation moment arms about the fulcrum 44. Assume that the ski length is 57 inches and that the fulcrum 44 is 27 inches from the rearward end. For simplicity we assume that the ski has a constant cross-sectional outline throughout its entire length, so that we have 30 inches of ski ahead of the fulcrum 44 and 27 inches behind, giving a flotation of 30K forward and of 27K rearwardly. The forward flotation can be considered the equivalent of a single upward force acting at a distance of 15 inches from the fulcrum 44, and the rearward flotation can be considered the equivalent of a single upward force acting at a distance of 13 /2 inches from the fulcrum 44. K is a constant related to the cross-section. Thus the forward flotation moment arm is 30K 15:450K, and the rearward flotation moment arm is 27K 13 /2=364K. The former thus exceeds the latter by 23.6%. We have ignored the distribution of the weight of the ski in the flotation moment arm calculation because it is so small by comparison.

For the weight moment arm, a similar calculation is carried out. Let us assume that by providing the spaces 40 the ski is balanced about thefulcrum 44 prior to adding the weight 46, and that the forward and rearward weight moment arms are each 4 pound-feet. The addition of a one-half pound weight (46) at a distance of 2.2 feet from the fulcrum 44 will increase the rearward weight moment arm by 1.1 pound-feet so that the rearward weight moment arm will exceed the forward weight moment arm by 27 /2 An alternative to adding the weight 46 as shown in FIG. 1 is to reduce the weight of the forward end of the ski by cutting away marginal portions of the piece 34 of rigid sheet material as shown by the dotted line 70- in FIG. 2. 'If the removed marginal portions are about 2 inches wide and 2 feet long, then the weight of material removed, assuming the sheet is inches plywood, would be about one pound, of which the centre of gravity is located about one foot from the fulcrum 44, thereby lessening the forward weight moment arm by about one poundfoot. Hence, this expedient causes the rearward weight moment arm to exceed the forward weight moment arm by about 33%.

It is further possible to provide, by vacuum-forming or other conventional technique, a hard, clear plastic coating or covering over the entire ski. A smooth-surface coating will not only protect the ski from abrasion and collision damage, but will reduce frictional drag in the water.

What I claim as my invention is:

1. A buoyant manoeuvrable ski comprising:

an elongated body portion of rigid foam plastic material, the body portion having a forward end, a rearward end, and a bottom surface free of all obstructions, the bottom surface curving upwardly at the forward end to provide a ski-tip,

foot-gripping means located between the ends of the body portion at a location rearwardly of the midflotation point such that the flotation moment forwardly of that location is from about to about 30% greater than the flotation moment rearwardly of that location,

the weight moment of the ski rearwardly of the said location being from about 20% to about 40% greater than the weight moment of the ski forwardly of the said location,

the ski having water-gripping means ensuring that the ski encounters less resistance to forward movement through water than to rearward movement through water.

2. The invention claimed in claim 1, in which said water-gripping means is a resilient flipper attached to said bottom surface part-way along the upwardly curving skitip and extending downwardly and rearwardly from its point of attachment to a free edge normally located below the plane of the bottom surface.

3. The invention claimed in claim 2, in which the said foot-gripping means is a foot-well extending downwardly from the top of the elongated body portion, the foot-well having a forward toe-grip extension into which the user can insert the front of his foot, and in which the ski has a constant cross-sectional outline rearwardly of the ski-tip, with the exception of said footwell.

4. The invention claimed in claim 3, in which the ski includes a reinforcing layer of rigid sheet material buried within the elongated body portion on a horizontal plane and extending substantially the full length of the ski, the reinforcing layer defining the bottom of the said footwell, the flipper being at least partly braced by the forward end of said reinforcing layer.

5. The invention claimed in claim 4, in which the flipper has an attachment edge for attaching to the ski, the attachment edge being parallel to said free edge and having a marginal portion bent at right-angles to define a flange, the body portion having a slot on said ski-tip in which said flange is received.

6. The invention claimed in claim 1, in which the bottom is substantially concavely curved and has substantially parallel side edges, to improve tracking when the user is being towed through the water.

7. The invention claimed in claim 1, in which the rearward end of the ski slopes downwardly and slightly rearwardly to define an acute angle of about with said bottom surface, and in which said water-gripping means includes spoon-like concave rabbets in said rearward end to improve the grip on the water when rearward pressure is applied to the ski.

8. The invention claimed in claim 1, in which the ski has a stabilizing fin extending downwardly from the rear of the bottom surface, the fin being aligned with the ski.

'9. The invention claimed in claim 2, in which the rigid foam plastic material is polystyrene, and in which the flipper is made of polypropylene.

10. The invention claimed in claim 2, in which the flipper is curved in a vertical plane parallel with the ski, the curvature being convex rearwardly, the flipper being attached to the ski along a horizontal line on the upwardly curving ski-tip, said horizontal line being located above the plane of the substantially flat bottom surface by a distance equal to about one-quarter of the depth of the ski, the said free edge being parallel to said horizontal line and being normally located below the plane of the substantially flat bottom surface by a distance equal to about one-quarter of the depth of the ski.

11. The invention claimed in claim 10, in which the flipper is capable of being forced to lie against the upwardly curving ski-tip, and when so lying has its free edge located approximately where the bottom surface begins to curve upwardly into the ski-tip.

12. The invention claimed in claim 1, in which the ski includes a weight buried in the body portion adjacent the rearward end to produce the greater weight moment rearwardly of the said location.

13. The invention claimed in claim 2, in which the elongated body portion is made up of a first part of U-shaped cross-section, and a second part of T-shaped cross-section adapted to be received stem-down in the first part, said foot-gripping means being a foot-well in the second part of T-shaped cross-section, the stem of the T-shaped cross-section forwardly of said foot-well being narrower than the inner dimension of the U-shaped cross-section, whereby air-spaces are left on either side of the stem forwardly of said foot-well, a reinforcing layer of rigid sheet material extending substantially the full length of the body portion on a horizontal plane and lying beneath the bottom of the stem of the said second part of T-shaped cross-section, said foot-well extending down to said layer of rigid sheet material, the bottom surface having substantially parallel side edges, to improve tracking when the user is being towed through the water.

14. The invention claimed in claim 13, in which the rearward end of the ski slopes downwardly and slightly rearwardly to define an acute angle of about 80 with said bottom surface, the water-gripping means further including spoon-like concave rabbets in the rearward end to improve the grip on the water when rearward pressure is applied to the ski, the ski further having a stabilizing fin aligned with the ski and extending downwardly from the rear of the bottom surface, the rigid foam plastic material being polystyrene and the flipper being made of polypropylene.

15. The invention claimed in claim 14, in which the flipper is curved in a vertical plane parallel with the ski, the curvature being convex rearwardly, the flipper being attached to the ski along a horizontal line on the upwardly curving ski-tip, said horizontal line being located above the plane of the bottom surface by a distance equal to about one-quarter of the depth of the ski, the said free edge being parallel to said horizontal line and being normally located below the plane of the bottom surface by a distance equal to about one-quarter of the depth of the ski, and in which the flipper is capable of being forced to lie against the upwardly curving ski-tip, and when so lying has its free edge located approximately where the bottom surface begins to curve upwardly into the ski-tip.

16. The invention claimed in claim 15, in which the ski includes a weight buried in the body portion adjacent the rearward end to produce the greater weight moment rearwardly of the said location, and in which the flipper has an attachment edge for attaching to the ski, the attachment edge being parallel to said free edge and having a marginal portion bent at right-angles to define a flange, the body portion having a slot on said ski-tip in which said flange is received, the flange being glued into said slot.

17. The invention claimed in claim 13, in which said reinforcing layer of rigid sheet material is wider rearwardly of said foot-well than it is forwardly thereof, thereby to produce the greater weight moment rearwardly of the said location.

References Cited UNITED STATES PATENTS 9/1971 Ia Rocca et al. 9--310 D 7/ 1916 Horn 9-310 D 5/1962 Klein 9310 A 3/1911 Hellmann 9310 D 4/1962 =Fortin 9310 A FOREIGN PATENTS 12/1936 Switzerland 9-3 10 D 1/ 1 932 France 9310 D 10/1933 France 9310 B 6/1911 Germany 9310 D 15 MILTON BUCHLER, Primary Examiner P. E. SAUBER'ER, Assistant Examiner 

